What Is Twilight?

Twilight is the faint
light which appears a little before sunrise and again after sunset before it
gets really dark. On the Moon there is no twilight. Darkness comes suddenly as
soon as direct sunlight ceases to reach the moon’s surface. This does not
happen on Earth because of the halo of air, called the atmosphere, which
surrounds it. When the Sun goes below the horizon, its light leaves the Earth
but is reflected downward from the upper atmosphere, illuminating the lower
atmosphere so that Earth’s surface is neither completely lit nor
completely dark. The word twilight is also used to denote the periods
of time when this illumination occurs.

The farther the Sun is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun reaches 18° below the horizon, the twilight’s brightness is nearly zero, and evening twilight becomes nighttime. When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the lit sky, twilight has long been popular with photographers, who sometimes refer to it as “sweet light”, and painters, who often refer to it as the blue hour, after the French expression l’heure bleue.

Poets and writers have
written of the evening twilight—or “the gloaming”, as it is called in
Scotland—as an enchanted time. Perhaps one reason is that familiar objects
become distorted in the half-light and we imagine we are seeing things that are
not really there.

Twilight should not be confused with auroras, which can have a similar appearance in the night sky at high latitudes. By analogy with evening twilight, the word twilight is also sometimes used metaphorically, to imply that something is losing strength and approaching its end. For example, very old people may be said to be “in the twilight of their lives”. The collateral adjective for twilight is crepuscular, which may be used to describe the behavior of insects, fish, and mammals that are most active during this period.

Content for this question contributed by Ada Abram, resident of Omaha, Douglas County, Nebraska, USA